Switching mechanical probe heads are primarily used when measuring a motor vehicle chassis with the aid of coordinate measuring apparatus for measuring the position of edges, bores, gap widths and the like. In contrast, for the measurement of the surface contours of such workpieces, optical measuring probe heads such as so-called triangulation probes are often also used. These triangulation probes supply a measurement signal proportional to the distance of the probe from the workpiece surface. Accordingly, time consuming probe exchange operations are necessary when the chassis is to be completely measured with the mechanical probe head attached to the measuring arm of the coordinate measuring apparatus being exchanged for an optical probe head and vice versa.
In addition, purely optical chassis measurement methods are also known. In these methods, larger areas of the workpiece are imaged and the workpiece image is processed with the methods of image processing. These evaluation methods are however very complex and offer only a limited precision of the coordinate measurement. Furthermore, a two-dimensional image sensor is required for this purpose.
Finally, it is also known to measure a chassis with a special optical probe head which emits a fanned light beam and the coordinate values of the workpiece surface including any edges and bores are obtained according to the light-intersection method. Such an optical probe head is, for example, known under the name "Perceptron-Contour-Sensor" and is described in a publication of the Perceptron Company entitled "MV-300 Machine Vision", page 11. This optical probe head also utilizes a two-dimensional image sensor in order to evaluate the shape of the light intersection at bores and edges. Furthermore, an additional rotational axis in the laser direction is required for the operation of this sensor so that the emitted fanned light can be adjusted perpendicularly to the edge to be measured. When an additional rotational axis is needed at this location, then the condition must be considered that optical probe heads are often attached to the measuring arm of the coordinate measuring apparatus by means of a so-called rotation-pivot unit so that spatially inclined workpiece features can be measured in a simple manner. With its two rotational axes, this rotation-pivot unit expands the coordinate measuring apparatus into a five-axis apparatus. However, a still further axis (a sixth axis) is needed for aligning a light intersection to an edge on the workpiece.
A further embodiment of the above-mentioned probe is disclosed in U.S. Pat. No. 4,822,163. The light intersection of the probe described there is wedge-shaped and extends in several directions whereby it is no longer necessary that the probe must be rotated about the sixth axis.
However, the last-mentioned probe requires also a two-dimensional image sensor and relatively complex image processing electronics.